This application relates to a variable gain amplifier (VGA) with low phase sensitivity to changes in gain.
VGAs are required in a radio receiver to allow the receiver to handle an input signal with a large dynamic range without causing saturation of components in the back-end of the receiver. This is achieved by variably amplifying weak and strong input signals to a common voltage.
Recently, there has been increasing interest in multiple input multiple output (MIMO) radios. A MIMO radio transceiver has multiple transmitters and multiple receivers that operate simultaneously at a common local oscillator frequency. A radio link that employs MIMO algorithms processes transmit and receive signals from multiple paths to increase the range of reception and/or transmission data rates. In order to maximize the benefit of multiple receivers for some MIMO algorithms, the insertion phase of each receiver path needs to be calibrated so that the received signals are processed with phase coherency across receiver paths. If the phase shift through the radio path can be made insensitive to changes in gain then continual recalibration can be avoided as the path gain is varied. Therefore, the VGA should ideally have constant phase shift over all its gain settings.